From what you say, the number of images should not make a difference. To confirm this, try saving a smaller test gallery (with just a few images) to the same location to see if the same problem occurs.
There is no known bug with regard to saving galleries so the problem may somehow be unique to your own system or images.

Please make sure that you are saving the gallery to a new empty folder.
Also, please check that the folder you are trying to save your gallery to is on your computer's local hard drive and not on an external or network drive.
Another thing to check is that the folder you are trying to save your gallery to is not synced to a file sharing service such as Dropbox or Google Drive.

Try creating a new empty folder on your desktop before using JuiceboxBuilder-Pro and then navigate to this folder when saving your gallery.

If this does not help, then try also ensuring that the source images are on your computer's local hard drive too (and not on an external or network drive).

It seems unlikely but perhaps there is a corrupt image which the program is hanging on whilst writing to your 'images' folder.
Try splitting your images into two batches and create a test gallery with each batch. If one gallery saves OK and the other one fails, then there may be a problem image.
You can then split the failing batch into two and repeat the process until you find the problem image.

I hope this helps or at least points you in the right direction.